Shape memory polymer (SMP) composite materials carry a ‘memory’ of their original shape, which they return to in response to a trigger such as light, heat, or a magnetic field. Usually, this type of material relies on nanoscale fillers such as gold nanorods, carbon nanotubes, or graphene to generate heat when exposed to light, which drives the shape change.

“In recent years, SMP composites have been increasingly used in medical applications, however they suffer from poor biodegradability and biocompatibility because of the nanofillers,” explains Xuefeng Yu, director of the Center for Biomedical Materials and Interfaces at Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, who led the research.

Together with Changshun Ruan and colleagues from Wuhan University, City University of Hong Kong, and Chongqing University, Yu has developed a composite based on polyurethane (PU) and black phosphorous (BP) – a stable, crystalline form of the two-dimensional semiconductor – that behaves like a SMP, can be remotely trigged by near-infrared (NIR) light, and is completely biodegradable.

“This composite not only possesses excellent NIR-photoresponsive shape memory performance but also good biodegradability and biocompatibility,” says Yu. “Using light to trigger the shape recovery process is particularly appealing because light can more easily control remote activation with higher spatial and temporal resolution compared to other stimuli, such as heat or a magnetic field.”

“Such an SMP composite enables the development of intelligent implantable devices, which can be easily controlled remotely by NIR light and degrade gradually after performing the designed function in the body,” explains Yu.

The composite is prepared simply by mixing a suspension of BP nanosheets, which have been produced by liquid exfoliation, with a PU solution. Then the mixture is solution cast into a mold and left to dry. The BP nanosheets act as photothermal nanofillers, absorbing NIR light and converting it into heat, driving a shape change in the thermo-responsive PU.

The BP/PU SMP composite recovers its original shape completely – more effectively than other common SMPs like hydrogels or polyetherurethane. Unlike other photothermal SMPs, when the composite degrades, it produces only nontoxic carbon dioxide, water and phosphate.

“This material enables the development of novel smart implantable devices which can be useful in many biomedical applications, especially for minimally invasive surgery,” Yu told Materials Today.

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